This invention relates to a circuit for the parallel coupling of audio frequency transmitting stations in a wave band filter construction, wherein the wave band filter has been designed to consist of two resonant circuits tuned to an audio frequency essentially free of the line harmonics, coupled magnetically to each other, and electrically separated, and wherein the line-side circuit has been designed as a series resonant circuit.
Central control stations are designed to transmit command impulses from a central command unit through the electrical mains system to all user terminals of the electrical system for the purpose of switching users on or off, or for any other type of control switching. Audio frequency pulses are fed into the system at the command unit, while the end users to be controlled are equipped with receivers responding to certain commands and executing the switching functions so commanded.
For the selective transmission of the audio frequency to the system to be controlled, and for the electrical separation of the system from the audio generator, coupling filters which are generally coupled in series or parallel are utilized. The type of coupling depends on the type of audio frequency used, unless additional measures are taken. Normally, the parallel type coupling is used for audio frequencies in excess of 300 Hz.
Known parallel couplings for audio frequency-type central control units, for each individual phase, consist of a line-side series circuit supplied with audio energy by the audio generator in the transmitting station via a transformer (isolating transformer). These types of couplings are simple, relatively inexpensive solutions, but are unsatisfactory because they usually cause significant repercussions on the audio generator side. Therefore, it has been suggested that these detrimental effects be reduced, as is usual in the case of series couplings, by means of a reactive network on the audio generator-side connected in parallel to the primary coil of the isolating transformer, wherein said reactive network preferably takes the form of a series circuit with an additional capacitance preferably connected in parallel to said reactive network and compensating for the audio frequency.
In another known parallel coupling, two series resonant circuits are used for similar reasons, one of said circuits being on the system side and one on the audio generator side, and wherein the coils of said circuits are connected in a transformer relationship, preferably coupled hypercritically, but electrically separated. Thereby, a remarkable degree of safety with regard to line repercussions on the audio generator is achieved, together with increased bandwidth and an increase in the audio frequency voltage in the vicinity of the resonant frequency, an increased damping of the first harmonic oscillation as well as the adjacent harmonic of the line frequency, plus a passive protective effect with the push-button protective switch of the audio generator in "open" position. Therefore, this circuit arrangement is especially useful with load-type static inverters in the audio generator.
The present invention concerns the problem of creating a simple circuit for the parallel coupling of audio frequency central-control stations or units of the type mentioned above, which circuit, while retaining the aforesaid advantages, features a higher degree of independence of any audio frequency voltage changes caused by changes in the line impedance, especially changes in the switching status of the system or extreme on-load switching operations.
The above problem is solved by the present invention in that the capacitor of the audio generator-side resonant circuit has been connected in parallel to the audio generator and to the coupling inductance of an ironless coupling transformer.